The present invention relates to an optical writing head such as an organic electroluminescence (EL) array exposure head, a method of manufacturing the same, and an image forming apparatus using the same. More particularly, the present invention relates to an optical writing head comprising ball lenses arranged corresponding to elements of a light emitting element array such as organic EL array or a light shatter element array for condensing fluxes of light from the elements onto a photoreceptor, a method of manufacturing the same, and an image forming apparatus using the same.
The present invention also relates to an organic EL array exposure head which is designed to prevent crosstalk between pixels, and an image forming apparatus using the same.
Conventionally, various techniques of using an organic EL array as an exposure head for image forming apparatus have been proposed. Those concerned therewith are as follows.
In Japanese Patent Unexamined Publication No. H10-55890, an organic EL array as a whole is fabricated on an insulating substrate made of glass, and a separate driver IC is combined with the organic EL array. A condensing rod lens array is used to condense light-emitting parts of the organic EL array to form an image on a photosensitive drum.
Japanese Patent Unexamined Publication No. H11-198433 uses a one-chip organic EL array having a plurality of rows. However, the optical system for condensing the light-emitting parts to form an image on a photosensitive drum is not clear. It should be noted that an EL layer of the organic EL array is deposited by evaporation.
In Japanese Patent Unexamined Publication No. 2000-77188, microlenses are formed on the top surface of a substrate by an ion exchange method. Alternatively, microlenses are formed on the bottom surface of a substrate by a method using a photoresist or by a replica method. An organic EL array having a resonator structure is deposited in alignment with the microlenses by evaporation.
Japanese Patent Unexamined Publication No. H10-12377 relates to a method of producing an active matrix organic EL display. An organic light-emitting layer is formed by the inkjet method over a glass substrate having thin-film transistors.
In Japanese Patent Unexamined Publication No. 2000-323276, a barrier plate is formed and coating is conducted by the inkjet method so as to form a hole injection layer and an organic light-emitting layer of an organic EL element.
In Japanese Patent Unexamined Publication No. 2001-18441, a light-emitting layer and a TFT layer for controlling light emission from the light-emitting layer are formed in a photosensitive drum. In this manner, a printer is manufactured.
On the other hand, various techniques of using a light emitting diode (LED) array or a liquid crystal shatter array, besides an organic EL array, as an exposure head for image forming apparatus have been also proposed. In these cases, a condensing rod lens array is generally used to condense fluxes of light from light-emitting parts of the LED array or from shutter parts of the liquid-crystal shutter array onto a photosensitive drum.
When an organic EL array is used as an exposure head of a printer such as an electrophotographic printer, in case of using a condensing rod lens array for condensing fluxes of light from light-emitting parts of the organic EL array or the LED array or from shutter parts of the liquid-crystal shutter array onto a photosensitive drum, the exposure head requires long optical path length and thus needs increase in size. In addition, since the condensing rod lenses are not arranged in a one-on-one relation to the light-emitting parts or the shutter parts, periodical optical irregularity occurs. Moreover, since the condensing rod lens requires advanced manufacturing technology, the increase in cost is inevitable. In case that the organic EL array is integrated with microlenses, there are problems such as a restriction in usable microlens materials.
In case of using a microlense array, though the microsenses are arranged in a one-on-one relation to light-emitting parts, such crosstalk in which light from a light-emitting part is incident on a pixel position through a microlens, not a microlens corresponding to the light-emitting part, e.g. a microlens adjacent to the corresponding microlens easily occurs, thus leading to the reduction in resolution.
The present invention was made to overcome the aforementioned problems of conventional techniques. The first object of the present invention is to provide a small optical writing head having long work distance with little crosstalk and which comprises ball lenses which are arranged in a one-on-one relation to elements of a light-emitting element array such as an organic EL array and an LED array or an optical shutter element array such as a liquid-crystal shutter array to condense fluxes of light from the elements onto an image carrier of a photoreceptor, a method of manufacturing the same, and an image forming apparatus using the same.
The second object of the present invention is to provide a small exposure head which comprises microlenses in a one-on-one relation to elements of an organic EL array to condense fluxes of light from the elements onto an image carrier such as a photoreceptor with little crosstalk, enough resolution and contrast.
The first object is achieved by providing an optical writing head for projecting fluxes of modulated light from light-emitting parts of a light-emitting element array or fluxes of modulated light transmitted through shutter parts of an optical shutter element array onto an image carrier to form a predetermined pattern on the image carrier, wherein the optical writing head is characterized by comprising ball lenses which are arranged such that the alignment of the ball lenses corresponds to the alignment of the light-emitting parts of the light-emitting element array or the shutter parts of the optical shutter element array.
In this case, it is preferable that the ball lenses are fixed to a transparent member on the surface of the light-emitting element array or the optical shutter element array by transparent adhesive.
It is also preferable that the diameter of the ball lenses is equal to or less than the alignment pitch of the light-emitting parts of the light-emitting element array or the alignment pitch of the shutter parts of the optical shutter element array.
It is also preferable that the refractive index of the ball lenses is equal to or more than the refractive index of the transparent adhesive.
It is also preferable that the thickness of the transparent member is equal to or less than the diameter of the ball lenses.
It is preferable that the optical writing head further comprises a light-shielding mask arranged among the ball lenses.
In this case, the light-shielding mask may be a light-shielding mask plate formed with holes corresponding to the array of the ball lenses, wherein at least either of the positioning and the fixing of the ball lenses is achieved by the light-shielding mask plate which is disposed on the output side of the ball lenses.
For example, the light-emitting element array may be an organic EL array.
The method of manufacturing the optical writing head of the present invention, which is for projecting fluxes of modulated light from light-emitting parts of a light-emitting element array or fluxes of modulated light transmitted through shutter parts of an optical shutter element array onto an image carrier to form a predetermined pattern on the image carrier and which comprises ball lenses which are arranged such that the alignment of the ball lenses corresponds to the alignment of the light-emitting parts of the light-emitting element array or the shutter parts of the optical shutter element array, is characterized in that the alignment of the ball lenses is achieved by forming a pattern of holes for positioning the ball lenses to positions corresponding to the light-emitting parts of the light-emitting element array or the shutter parts of the optical shutter element array on a transparent member on the surface of the light-emitting element array or the optical shutter element array, and fitting the ball lenses into the holes.
In this case, the pattern of holes may be composed of a member which also functions as a light-shielding means.
The present invention also pertains to an image forming apparatus using an optical writing head as described above as an exposure head for writing an image on an image carrier.
An example of such image forming apparatus is a color image forming apparatus of a tandem type having at least two image forming stations each comprising a charging means, an exposure head, a developing means, and a transfer means which are arranged around an image carrier, wherein the color image forming apparatus forms a color image by passing a transfer medium through the respective stations.
An organic EL array exposure head of the present invention for achieving the second object comprises a long substrate and an array of organic EL elements aligned just like pixels aligned in at least of one row and is characterized by comprising a barrier plate formed with optical holes for preventing crosstalk of light, i.e. a phenomenon that fluxes of light emitted from the light-emitting parts of adjacent organic EL elements are immixed at the condensing position of light from the light-emitting part of either organic EL element, wherein the barrier plate is arranged on the output side of the array of the organic EL elements.
In this case, it is preferable that plus lenses are arranged at positions of the holes of the barrier plate, respectively.
It is also preferable that the coefficient of linear expansion of the substrate and the coefficient of linear expansion of the barrier plate are substantially equal to each other.
The barrier plate may be a metal plate formed with holes or a barrier plate made of a resin with holes which is formed by molding.
The barrier plate and the substrate may be integrally formed.
The level of the output-side surface of the barrier plate may be higher or lower than the level of the plus lenses.
It is preferable that the inner surface of the each hole of the barrier plate has light reflectivity or light absorptivity.
The plus lenses may be made of a resin. In this case, the plus lenses are formed within the holes of the barrier plate by the inkjet method or by a molding (replica) method
Further, the plus lenses may be made of glass.
Furthermore, the plus lenses may be ball lenses.
In addition, the each organic EL element may be either of cases of emitting light from its anode side and emitting light from its cathode side.
Moreover, the each organic EL element may be of a polymer type or of a low-molecular type.
It is preferable that the light emission of the each organic EL element is controlled by a TFT disposed on the substrate.
The present invention also pertains to an image forming apparatus using an organic EL array exposure head as described as an exposure head for writing an image on an image carrier.
An example of such image forming apparatus is a color image forming apparatus of a tandem type having at least two image forming stations each comprising a charging means, an exposure head, a developing means with toner, and a transfer means which are arranged around an image carrier, wherein the color image forming apparatus forms a color image by passing a transfer medium through the respective stations.
In case of an image forming apparatus of a type developing an image by a developing means with toner, it is preferable to satisfy both the following relations:
                                                                  Φ              T                        -                          Φ              L                                                ≤                  0.2          ⁢                                          ⁢          eV                                    (        1        )                                                                              Φ              T                        -                          Φ              B                                                ≥                  0.5          ⁢                                          ⁢          eV                                    (        2        )            wherein ΦL is the work function of the material for the plus lenses, ΦB is the work function of the material of the barrier plate, and ΦT is the work function of the material of toner.
In the optical writing head of the present invention achieving the first object, the ball lenses are arranged such that the alignment of the ball lenses corresponds to the alignment of the light-emitting parts of the light-emitting element array or the shatter parts of the optical shutter element array, thereby preventing the occurrence of periodical optical irregularity which has been conventionally occurred due to use of condensing rod lenses. In addition, the reduction in size of the optical writing head can be achieved and enough work distance can be ensured.
Further, in the optical writing head of the present invention achieving the second object, the barrier plate, having optical holes for preventing crosstalk of light i.e. a phenomenon that fluxes of light emitted from the light-emitting parts of adjacent organic EL elements are immixed at the condensing position of light from the light-emitting part of either organic EL element, is provided on the light-emitting side of the array of organic EL elements, thereby reducing the crosstalk between adjacent pixels and thereby obtaining enough resolution and contrast.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.